In a manufacturing process of an electronic device such as a semiconductor device, plasma processing of a workpiece may be performed using a plasma processing apparatus, and plasma etching is one kind of plasma processing. A resist mask used for plasma etching is formed by a photolithography technique, and a critical dimension of a pattern formed in an etching target layer depends on the resolution of the resist mask formed by the photolithography technique. However, the resolution of the resist mask has a resolution limit. There is an increasing demand for higher integration of electronic devices, and it is now required to form patterns with dimensions smaller than the resolution limit of resist mask. Therefore, as described in Patent Document 1, a technique of adjusting the dimensions of a resist mask by forming a silicon oxide film on the resist mask, and reducing the width of an opening provided by the resist mask has been proposed.